Move Compilation::PrintJob and PrintDiagnosticJob into Job::Print.

This moves the code to Job.cpp, which seems like a more natural fit,
and replaces the "is this a JobList? is this a Command?" logic with
a virtual function call.

It also removes the code duplication between PrintJob and
PrintDiagnosticJob and simplifies the code a little.

There's no functionality change here, except that the Executable is
now always printed within quotes, whereas it would previously not be
quoted in crash reports, which I think was a bug.

Differential Revision: http://llvm-reviews.chandlerc.com/D1653